Method of electrodepositing luminescent material on insulating substrate



Fig. I.

' f Harold D. Wilcox an m Donald M. Phillips United States Patent O 3,525,679 METHOD F ELECTRODEPOSITING LUMI- NESCENT MATERIAL 0N INSULATING SUBSTRATE Harold D. Wilcox, South Port, and Donald M. Phillips,

Cayuta, NX., assignors to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed May 5, 1964, Ser. No. 364,908 -A Int. Cl. B01k 5/02; C23b 13/00 U.S. Cl. 204-181 11 Claims ABSTRACT 0F THE DISCLOSURE The invention relates to depositing a conductive coating on an insulating substrate to provide adequate com ductivity for electrophoretically depositing a powdered material such as a luminescent material onto the conductive coating and then chemically removing a substantial portion of the conductive coating through the luminescent coating to provide a coating of luminescent material on the insulating substrate.

This invention relates to a method of depositing'a coating of a powdered material on a nonconductive surface.

One particular application of this invention is in the application of a luminescent coating on the face plate of a cathode ray tube. There are many methods of depositing a layer of phosphor or luminescent material on the inner surface of the face plate but this particular application is directed to providing a high resolution type of luminescent screen. The luminescent material is deposited by electrophoresis to provide the high resolution scre'en. In electrophoresis, an electrically conductive coating is required to serve as one electrode in the process and is provided on the inner surface of the face plate. The face plate is of an insulating material, such as glass, transmissive to radiations emitted by the luminescent screen.

Generally, the conducting film provided on the face plate and onto which the phosphor is deposited by the electrophoretic process is of a material such as tin oxide or gold. If this conductive material is left on the face plate, it is found that the transmission fo the light emitted from the phosphor through the face plate is reduced by to 35 percent. Such a reduction is unavoidable due to the thickness of the conductive layer required to provide the necessary resistance levels required for electrophoretic coating. Some conductive surfaces also have other undesirable properties such as providing a nonuniform coating. Some coatings also cause color shift and resultant transmission loss during subsequent heat treatment of the device. The color shift is found with gold conductive surfaces. Gold does provide a particularly good conductive coating for this application in other respects in that it provides the necessary flatness and uniformity required for high resolution. The difficulty found in duplicating the transmission and color shift of the gold film during subsequent bake cycles makes gold an unsatisfactory material for production. The gold layer also presents an unsightly defect as the color shift may be nonuniform over the face of the tube.

It is, accordingly, a general object of this invention to provide an improved method of depositing a coating of a powdered material on a substrate.

It is another object of this invention to provide an irnproved method of depositing a luminescent material by electrophoretic means on a substrate by providing a conductive layer on the substrate and then chemically removing a portion of conductive substrate after the electrophoretic deposition.

Briefly, the present invention accomplishes the above- ICC cited objects by depositing a conductive coating on an insulating substrate to provide adequate conductivity for electrophoretically depositing a powdered material onto the conductive coating and then chemically removing a substantial portion of the conductive coating to provide a coating of the material on the insulating substrate.

Further objects and advantages of the invention will become apparent as the following description proceeds and features of novelty which characterize the invention will be pointed out in particularity in the claims annexed to and forming a part of this specification.

For a better understanding of the invention, reference may be had to the accompanying drawings, in which:

FIG. l shows a partially sectionalized cathode ray tube in connection with which the present invention may be usefully applied; and

FIGS. 2 to 4 inclusive are enlarged views, in section, illustrating various steps in the process embodying the invention.

Referring now to the drawing, there is illustrated in FIG. 1 a cathode ray tube of the type in which the invention may be applied with particular advantage. The tube as illustrated includes an envelope 10 having an enlarged flared portion 12, a neck portion 14 and a face plate portion 16. An electron gun 20 is provided within the neck portion 14 for generating an electron beam which is directed onto the face plate 16. The face plate 16 provides a substantially transparent window to the radiations emitted from the screen. A layer 24 of a luminescent material which is adapted to be excited by electron bombardment to produce visible light is provided on the inner surface of the face plate 16. A conductive coating 26 is provided on the inner surface of said luminescent layer 24 for both improving the light output from the screen 24 as well as preventing ion burn of the luminescent mate rial in layer 24 and providing an electrode for accelerating the electrons from the gun 20. A suitable deflection system 28 may also be provided about the neck portion 14 of the envelope 10 for deflecting the electron beam to scan a raster over the luminescent screen 24. This invention is directed to the process of providing the screen structure and the remainder of the cathode ray tube is well-known in the art.

In accordance with our invention, the luminescent layer 24, which may be of suitable materials such as zinc sulfide (P-11), zinc cadmium silicate (P-20) or zinc magnesium silicate (P-16), is applied to the inner surface of the face plate 16. In the first step of this operation, the transparent substrate 16 which may be of a material such as glass is cleaned by well known techniques preparatory to the deposition of a suitable conductive layer 32 of a material such as gold or indium onto the face plate 116. The face plate 16 is normally sealed to the envelope 10 by methods Well known in the art after the luminescent layer 24 is applied.

The conductive layer 32 may be applied by several techniques. It is only necessary that it have a resistance of about ohms/square. The gold layer may be sputtered or evaporated onto the face plate, The thickness of the conductive layer is relatively unimportant in that the gold layer 32 is removed after the phosphor is electrophoretically coated.

In one specific process, the conductive layer of gold 32 may be deposited in the following manner. The face plate 16 is positioned within a system and the system is evacuated to a pressure of about 1 l05 torr. With the face plate 16 having a diameter of about 5 inches, 5 milligrams of aluminum are positioned within an open Crucible at a distance of about 19 inches from the face plate 16 and the aluminum is evaporated to provide a layer 30 of a few angstroms in thickness. The aluminum layer 30 provides better adherence of the gold to the glass face plate 16. After the coating 30 is applied, a crucible containing about 70 milligrams of gold is positioned within the evacuated chamber at about 19 inches from the face plate 16. The layer 32 of gold is evaporated onto the layer 30. The gold layer 32 is also only a few angstroms in thickness but thicker than the layer 30. After the evaporation of the gold layer 32, a protective coating 34 of a suitable material such as indium is evaporated onto the gold layer 32 by placing about 3 milligrams of indium in a crucible at a distance of 19 inches from the face plate and evaporated at a pressure of 1 l05 torr. The layer 34 protects the gold layer 32 and makes it thermally stable. A reading of the transmission of radiations of wavelength 5,000 angstroms through the conductive layers 30, 32 and 34 and the glass substrate 16 is about 75%. The resistance of this resulting substrate of layers 30, 32 and 34 is about 100 ohms/square and provides an excellent conductive electrode for the electrophoretic deposition of the luminescent layer in the next step. The structure is shown in FIG. 2. It should again be noted that the layers 30 and 34 are not necessary but improve the results.

The next step of the process is accomplished by immersing the face plate 16 with the conductive coatings 30, 32 and 34 beneath a surface of a suitable phosphor bath. The phosphor bath consists of about grams of a suitable luminescent or phosphor material such as zinc sulfide (P-11), about 200 milligrams of a suitable electro lyte such as thorium nitrate (Th(NO3)4) slurried in about 900 milliliters of ethyl alcohol. A suitable coating 36 of phosphor of about 4 microns in thickness is provided on the conductive layer 32 in a time period of about l0 seconds with a voltage of 150 volts D.C. applied. The other electrode in this process may be of a suitable material such as stainless steel or carbon. The structure is shown in FIG. 3.

In those applications Where brightness is not desired, the conductive substrate could remain consisting of layers 30, 32 and 34. However, in this application the conductive substrate is removed. A small amount of about .1 to percent of potassium cyanide solution is then placed over the screen with just enough KCN to cover the entire screen. The KCN solution is allowed to soak on the screen for approximately 4 minutes after which the KCN is removed and the screen substrate flushed with deionized water. This KCN soak and flush treatment may be repeated as many times as necessary in order to remove substantially all traces of the gold layer 32. This usually requires three to four treatments for a total KCN soak time of about 10 to 15 minutes.

It is necessary to limit the strength of the KCN solution so as to not destroy the screen structure. The repeat soak and liush treatment permits the KCN solution to convert the solid gold layer into a compound solution. The gold in solution may be removed through the porous like phosphor layer 24. In this manner, a substantial portion of gold layer 32 and the other layers 30 and 34 will be removed to the point that transmission of light will not be substantially absorbed or reiiected by the conductive substrate of layers 30, 32 and 34 used for the electrophoretic process. By repeated KCN treatment, the transmission of light through the substrate 16 is returned to that prior to deposition of conductive layers 30, 32 and 34. The bond between the particles of the phosphor layer 24 to each other and to the glass face plate 16 is believed due to electrostatic charges. The structure is illustrated in FIG. 4.

The face plate 16 and the funnel assembly is then mated together and sealed by methods well known to the art. The bulb may then be provided with an aluminum coating 26. The aluminum coating 26 may be applied in the known conventional manner in which an organic film is provided on the phosphor layer 24 and then the aluminum evaporated onto the organic ilm and the organic 4 ilm removed during the normal bake-out which is for about 10 minutes at 410 C.

While there have been shown and described what are presently considered to be the preferred embodiments of the invention, modifications thereto lwill readily occur to those skilled in the art. It is not desired, therefore, that the invention be limited to the specific method described and it is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.

We claim as our invention:

1. The method of producing a smooth thin coating of a powdered material on an insulating substrate which comprises the steps of depositing a substrate of electrically conductive material onto said insulating substrate, depositing a thin continuous coating of said powdered material onto said conductive substrate, and chemically removing a substantial portion of said conductive substrate through said thin coating of powdered material to provide said powdered material coating on said insulating substrate.

2. The method of producing a smooth thin coating of a powdered luminescent material on a substrate which comprises the steps of depositing a coating of an electrically conductive material on said substrate, depositing a thin coating of said luminescent material onto said conductive coating by electrophoresis, and chemically removing a portion of said conductive coating through said thin coating of luminescent material to provide said luminescent coating on said substrate.

3. The method of producing a luminescent screen comprising the steps of depositing a coating of an electrically conductive material onto a screen substrate, depositing a thin coating of luminescent material onto said conductive coating by electrophoresis, chemically converting a portion of said conductive coating to a compound solution and removing said solution through said thin coating of luminescent material.

4. The method of producing a luminescent screen comprising the steps of depositing a conductive material onto a glass support, depositing a thin coating of luminescent material onto said conductive material by electrophoresis and chemically removing a portion of said conductive material through said thin coating of luminescent material to permit substantially all of light emission from said luminescent material to be transmitted through said glass support.

5. The method of producing a luminescent screen comprising the steps of depositing a coating of gold onto a. face plate, depositing a thin coating of luminescent material onto said gold coated face plate by electrophoresis and removing said gold through said thin coating of luminescent material by treating said screen with one of the compounds selected from the group consisting of lithium cyanide, sodium cyanide, potassium cyanide and ammonium cyanide.

6. The method of producing a smooth thin coating of a powdered phosphor material onto an insulating substrate which comprises the steps of evaporating a conductive coating including gold onto said substrate, depositing a thin coating of said phosphor onto said conductive coating, and chemically removing a substantial portion of said conductive coating through said thin coating of phosphor to provide said phosphor coating on said insulating substrate.

7. The method of producing a smooth thin coating of p 8. The method of providing a smooth thin coating of a powdered luminescent material by electrophoresis on the face plate of a cathode ray tube wherein the transmission of the light emitted by said luminescent material is limited only by the face plate, comprising the steps of depositing a coating of an electrically conductive material on said face plate, depositing a thin coating of said luminescent material onto said conductive coating by electrophoresis, and chemically removing said conductive coating through said thin coating of said luminescent material to permit transmission of light from said luminescent material directly through said face plate.

9. The method of producing a luminescent screen comprising the steps of depositing a conductive coating including gold onto a screen substrate, depositing a thin coating of luminescent material onto said conductive coating by electrophoresis, contacting said screen with a dilute cyanide solution having a concentration of .1 to 25 percent to convert a portion of said conductive coating to a soluble compound and removing said soluble compound through said thin coating of luminescent material.

10. The method of producing a luminescent screen comprising the steps of depositing a conductive coating comprised of gold onto a substrate, depositing a thin coating of luminescent material onto said conductive coating substrate by electrophoresis and removing said gold through said thin coating of luminescent material by treating said screen `with one of the compounds selected from the group consisting of lithium cyanide, sodium cyanide, potassium cyanide and ammonium cyanide.

1.1. The method of producing a luminescent screen comprising the steps of depositing an electrically conductive coating comprised of gold onto a screen substrate, depositing a thin coating of luminescent material onto said conductive coating by electrophoresis, repeatedly contacting said screen with a dilute cyanide solution having a concentration less than 25 percent and rinsing said screen to remove a substantial portion of said conductive coating by converting a portion of said conductive vcoating to a soluble compound and removing said soluble compound through said thin coating of luminescent material.

References Cited UNITED STATES PATENTS 3,267,013 8/1966 Mathias et al. 204-146 3,314,871 4/1967 Heck et al 204-181 FOREIGN PATENTS 215,847 7/ 1958 Australia.

OTHER REFERENCES Immendorfer, M.: Applying Photoconductive Elements on Nonconductive Substrates, in I.B.M. Technical Disclosure Bulletin, vol. `6, No. 6, p. 77, November 1963.

Kushner: Modern Gold Plating, in Products Finishing, pp. -56, January 1942.

HOWARD s. WILLIAMS, Primary Examiner Us. C1. X.R, 117-335, 31a- 9g 

